In a nuclear power plant such as a nuclear generation facility, it is an important issue to suppress emission of radioactive gas, particularly radioactive iodine composed of an organic iodine compound whose main component is molecular iodine (129I2, 131I2, 133I2, hereinafter, simply “I2”) and methyl iodide (CH3129I, CH3131I, CH3133I, hereinafter, simply “CH3I”) to the air, from a standpoint of preserving the ambient environment.
Conventionally, to prevent emission of radiation to the air from a nuclear power facility such as a nuclear generation facility at the time of an accident or the like, an iodine filter including an iodine adsorbent that adsorbs radioactive iodine is provided in an air-conditioning and ventilating system of the nuclear power facility, so that radioactive iodine contained in the gas is removed.
As the iodine adsorbent, for example, KI-impregnated activated carbon in which activated carbon is impregnated with kalium iodide (KI), Ag zeolite in which zeolite is impregnated with silver (Ag), and TEDA carbon in which activated carbon is impregnated with triethylene diamine (TEDA) can be mentioned (see, for example, Patent Literature 1). By using the KI-impregnated activated carbon as the iodine adsorbent, radioactive iodine is isotopically replaced by non-radioactive iodine of the KI-impregnated activated carbon, thereby collecting radioactive iodine. By using the Ag zeolite as the iodine adsorbent, radioactive iodine reacts with Ag of Ag zeolite to become silver iodide (AgI), thereby collecting radioactive iodine. By using the TEDA carbon as the iodine adsorbent, TEDA reacts with radioactive iodine to remove radioactive iodine.
In a nuclear power facility, to reduce emission of radioactive iodine contained in gas into the air as much as possible by an iodine filter, not only at the time of occurrence of an accident but also at the time of a steady operation, a radioactive iodine removal apparatus including an iodine adsorbent that removes radioactive iodine is provided also in the air-conditioning and ventilating system in a reactor building, an auxiliary building, and the like. Hygroscopic moisture in flue gas from the air-conditioning and ventilating system depends on the weather condition of the ambient air, and when the humidity in the ambient air is high such as a case of rain, the relative humidity of flue gas from the air-conditioning and ventilating system may reach up to 90%. When the ambient air is dry, the relative humidity of flue gas in the air-conditioning and ventilating system decreases to a very low level.
Furthermore, in a system having an air handling unit, for example, a cooling coil in the air-conditioning and ventilating system, flue gas from the air-conditioning and ventilating system is cooled by the air handling unit. Therefore, the relative humidity of flue gas increases at an outlet of the air handling unit. When flue gas from the air-conditioning and ventilating system is used for heating, on the contrary, the temperature of flue gas from the air-conditioning and ventilating system increases, and the relative humidity of flue gas decreases. The ambient air may be directly supplied without being treated by the air handling unit.
As described above, the radioactive iodine removal apparatus provided in the air-conditioning and ventilating system handles flue gas under broad humidity conditions with a relative humidity of 0% to 100%. The conventionally used iodine adsorbent is likely to be affected by the humidity in flue gas, and when the humidity in flue gas is high, the iodine removal performance of the iodine adsorbent decreases, and thus radioactive iodine cannot be removed stably and with high efficiency.
There is a proposal of an adsorbent in which a carrier such as alumina is impregnated with silver nitrate or silver sulfate and an organic layer is provided on the surface thereof, as an iodine adsorbent that suppresses a decrease in the radioactive iodine removal performance even if the humidity in gas containing radioactive iodine is high (see, for example, Patent Literature 2).